The energy spectrum of anomalous X-ray pulsars and soft gamma-ray repeaters

• Published in: Astronomy and astrophysics (Berlin) 2010-07, Vol.518 (2), p.A46
• Main Authors: Trümper, J. E., Zezas, A., Ertan, Ü., Kylafis, N. D.
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; pulsars: individual: 1E 1841–045; pulsars: individual: 1RXS J1708–4009; pulsars: individual: 4U 0142+61; stars: magnetic fields; X-rays: stars
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/200911834

Context. Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) exhibit characteristic X-ray luminosities (both soft and hard) of around 1035 erg s-1 and characteristic power-law, hard X-ray spectra extending to about 200 keV. Two AXPs also exhibit pulsed radio emission. Aims. Assuming that AXPs and SGRs accrete matter from a fallback disk, we attempt to explain both the soft and the hard X-ray emission as the result of the accretion process. We also attempt to explain their radio emission or the lack of it. Methods. We test the hypothesis that the power-law, hard X-ray spectra are produced in the accretion flow mainly by bulk-motion Comptonization of soft photons emitted at the neutron star surface. Fallback disk models invoke surface dipole magnetic fields of 1012 - 1013 G, which is what we assume here. Results. Unlike normal X-ray pulsars, for which the accretion rate is highly super-Eddington, the accretion rate is approximately Eddington in AXPs and SGRs and thus the bulk-motion Comptonization operates efficiently. As an illustrative example we reproduce both the hard and the soft X-ray spectra of AXP 4U 0142+61 well using the XSPEC package compTB. Conclusions. Our model seems to explain both the hard and the soft X-ray spectra of AXPs and SGRs, as well as their radio emission or the lack of it, in a natural way. It might also explain the short bursts observed in these sources. On the other hand, it cannot explain the giant X-ray outbursts observed in SGRs, which may result from the conversion of magnetic energy in local multipole fields.